The present invention is directed to structures having a plurality of discrete insulated elongated electrical conductors projecting from a support surface which are useful as probes for testing of electrical interconnections to electronic devices, such as integrated circuit devices and other electronic components and particularly for testing of integrated circuit devices with rigid interconnection pads and multi-chip module packages with high density interconnection pads and the apparatus for use thereof and to methods of fabrication thereof.
Integrated circuit (IC) devices and other electronic components are normally tested to verify the electrical function of the device and certain devices require high temperature burn-in testing to accelerate early life failures of these devices. Wafer probing is typically done on a single chip site at temperatures ranging from 25 C.-125 C. while burn-in is typically done on diced and packaged chips at temperatures ranging from 80 C. to 150 C. Wafer probing and IC chip burn-in at elevated temperatures of up to 200 C. has several advantages and is becoming increasingly important in the semiconductor industry. Simultaneous testing of multiple chips on a single wafer has obvious advantages for reducing costs and increasing production throughput and is a logical step towards testing and burn-in of an entire wafer.
The various types of interconnection methods used to test these devices include permanent, semi-permanent, and temporary attachment techniques. The permanent and semi-permanent techniques that are typically used include soldering and wire bonding to provide a connection from the IC device to a substrate with fan out wiring or a metal lead frame package. The temporary attachment techniques include rigid and flexible probes that are used to connect the IC device to a substrate with fan out wiring or directly to the test equipment.
The permanent attachment techniques used for testing integrated circuit devices such as wire bonding to a leadframe of a plastic leaded chip carrier are typically used for devices that have low number of interconnections and the plastic leaded chip carrier package is relatively inexpensive. The device is tested through the wire bonds and leads of the plastic leaded chip carrier and plugged into a test socket. If the integrated circuit device is defective, the device and the plastic leaded chip carrier are discarded.
The semi-permanent attachment techniques used for testing integrated circuit devices such as solder ball attachment to a ceramic or plastic pin grid array package are typically used for devices that have high number of interconnections and the pin grid array package is relatively expensive. The device is tested through the solder balls and the internal fan out wiring and pins of the pin grid array package that is plugged into a test socket. If the integrated circuit device is defective, the device can be removed from the pin grid array package by heating the solder balls to their melting point. The processing cost of heating and removing the chip is offset by the cost saving of reusing the pin grid array package.
The most cost effective techniques for testing and burn-in of integrated circuit devices provide a direct interconnection between the pads on the device to a probe sockets that is hard wired to the test equipment. Contemporary probes for testing integrated circuits are expensive to fabricate and are easily damaged. The individual probes are typically attached to a ring shaped printed circuit board and support cantilevered metal wires extending towards the center of the opening in the circuit board. Each probe wire must be aligned to a contact location on the integrated circuit device to be tested. The probe wires are generally fragile and easily deformed or damaged. This type of probe fixture is typically used for testing integrated circuit devices that have contacts along the perimeter of the device. This type of probe is also much larger than the IC device that is being tested and the use of this type of probe for high temperature testing is limited by the probe structure and material set. This is described with reference to applicant""s co-pending U.S. application Ser. No. 08/754,869 filed on Nov. 22, 1996, the teaching of which is incorporated herein by reference.
Another technique used for testing IC devices comprises a thin flex circuit with metal bumps and fan out wiring. The bumps are typically formed by photolithographic processes and provide a raised contact for the probe assembly. The bumps are used to contact the flat or recessed aluminum bond pads on the IC device. An elastomer pad is typically used between the back of the flex circuit and a pressure plate or rigid circuit board to provide compliance for the probe interface. This type of probe is limited to flexible film substrate materials that typically have one or two wiring layers. Also, this type of probe does not provide a wiping contact interface to ensure a low resistance connection.
The aluminum bond pads on a high density IC device are typically rectangular in shape and are recessed slightly below the surface of the passivation layer. If the wiping action of the high density probe is not controlled, the probe contact may move in the wrong direction and short to an adjacent aluminum bond pad or the probe contact may move off of the aluminum bond pad onto the surface of the passivation layer and cause an open connection.
The position of the probe tips must be controlled to ensure accurate alignment of the probes to the interconnection pads on the IC device. During high temperature burn-in testing, the thermal expansion mismatch between the probe structure and the IC device must be small to ensure that the probe position does not vary significantly over the burn-in temperature range. Thermal expansion mismatch within the probe structure can result in contact reliability problems.
The challenges of probing a single high density integrated circuit device are further multiplied for multi-chip and full wafer testing applications. Probe fabrication techniques and material selection are critical to the thermal expansion and contact alignment considerations. A small difference in the thermal expansion of the substrate, wafer, and probe construction will cause misalignment of the probe tip to the wafer contact pad. Compliance of the probe structure is another critical factor. Slight variations in the wafer metalization, warpage of the wafer, and slight variations in the probe height contribute to the total compliance requirements for the probe structure.
As the processing power of IC devices increases, the number of I/O and speed of the I/O signals increases to meet this need. The use of high speed signals and high density connections on an integrated circuit device provides an increased challenge to accurately test the function of the device. High inductance of the test probes and cross talk between probes can severely limit the ability to test high speed and high density chip connections. The inductance of the test probe can be reduced by reducing the probe length or by providing a probe structure that has a shield ground. The integral shielding also helps to reduce the cross talk between high density probes and reduces the need to place grounded probes between signal probes.
U.S. Pat. No. 5,177,439, issued Jan. 5, 1993 to Liu et al., is directed to fixtures for testing bare IC chips. The fixture is manufactured from a silicon wafer or other substrate that is compatible with semiconductor processing. The substrate is chemically etched to produce a plurality of protrusions to match the I/O pattern on the bare IC chip. The protrusions are coated with a conductive material and connected to discrete conductive fanout wiring paths to allow connection to an external test system. The probe geometry described in this patent does not provide a compliant interface for testing the aluminum bond pads on the IC device and does not provide a wiping contact interface. The substrate used for fabrication of this probe fixture is limited to semiconductor wafers which are relatively expensive. The high density probe with controlled wipe can be fabricated on a variety of inexpensive substrate with the fanout wiring.
Applicant""s co-pending U.S. application Ser. No. 08/754,869 filed on Nov. 22, 1996, now U.S. Pat. No. 5,821,763, the teaching of which is incorporated herein by reference describes a high density test probe for integrated circuit devices. The probe structure described in this docket uses short metal wires that are bonded on one end to the fan out wiring on a rigid substrate. The wires are encased in a compliant polymer material to allow the probes to compress under pressure against the integrated circuit device. The wire probes are sufficiently long and formed at an angle to prevent permanent deformation during compression against the integrated circuit device.
It is the object of the present invention to provide a probe for testing integrated circuit devices and other electronic components that use rigid bond pads for the interconnection means.
Another object of the present invention is to provide a probe structure that is an integral part of the fan out wiring on the test substrate or other printed wiring means to minimize the electrical conductor length as well as the contact resistance of the probe interface.
A further object of the present invention is to provide a probe with a compliant interface to compensate for slight variations in the rigid bond pad heights on the IC device and variations in the height of the probe contacts.
An additional object of the present invention is to provide a raised probe tip for contacting recessed surfaces on the IC device.
Yet an another object of the present invention is to provide a probe structure that has low inductance and low cross talk electrical properties.
Yet a further object of the present invention is to provide a probe structure that has an improved true position tolerance.
Yet an additional object of the present invention is to provide a probe structure that can be used for high performance and high frequency single chip or multiple chip wafer testing.
A broad aspect of the present invention is a structure having a substrate having a surface; a plurality of elongated electrical conductors extending away from the surface; the elongated electrical have a dielectric coating; and each of the elongated electrical conductors having a first end affixed to the surface and a second end projecting away from the surface.
A more specific aspect of the structure according to the present invention includes a coating of an electrically conductive material disposed on the dielectric coating.
Another more specific aspect of the structure according to the structure of the present invention includes a means for electrically interconnection the electrically conductive coating on at least a part of said plurality of elongated electrical conductors.
Another more particular aspect of the present invention is an apparatus for using the structure to test an electronic device having a means for holding the structure, means for retractably moving the structure towards and away from the electronic device so that the second ends contact electrical contact locations on the electronic device and substrate.
Another broad aspect of the present invention is a method of providing a substrate having a surface; providing a plurality of elongated electrical conductors each having a first end and a second end; bonding each of the first ends to the surface so that the second ends are disposed away from the surface; forming a dielectric coating on the elongated electrical conductors.
Another more specific aspect of the method according to the present invention further includes forming a coating of an electrically conductive material on the dielectric coating.